Monday-Anxiety from hell

I had such a nice Sunday.  A friend from work whose dad had knee surgery and can't ride invited me to help her exercise their horses on Sunday morning.  It was really nice out and he has great horses.  I got a little sunburned but it was awesome.  In the afternoon my boyfriend came over and put in drip irrigation on my patio and bought me new flowers(since he had let the ones at the house die) so my patio would be homey.  I felt very happy Sunday afternoon. 
Of course that is when it all has to fall apart.  I could not sleep..up most of the night.  Anxiety started this morning to the point I started to wonder if I could be sick. 
The usual thoughts start to go through my head...was I near anyone who was sick?  Did I cook my food well enough? or was it just bad?  Came to work anyway...more than likely it is just anxiety so no sense in staying at home thinking about it.  If if turns out I'm actually sick I can always go home. 
Why can I not even have a good day without everything going to shit?